1. Field of the Invention
The present invention relates to an image forming technique of forming an image based on image data by using an image processing section having an exposure unit which scans and exposes an image carrier, and a developing unit which visualizes, on a printing medium, an electrostatic latent image formed on the image carrier by scanning exposure of the exposure unit.
2. Description of the Related Art
These days, tandem type color image forming apparatuses are popular. The color image forming apparatuses of this type comprises developing units and photosensitive drums equal in number to coloring materials in order to increase the image forming speed of an electrophotographic color image forming apparatus. This color image forming apparatus sequentially transfers images of different colors onto an image convey belt or printing medium. It has already been known that the tandem type color image forming apparatus has a plurality of factors which cause a registration error. Various measures against the respective factors have been proposed.
One of the factors is the nonuniformity and mounting position error of the lens of a deflection scanning device, and the mounting position error of the deflection scanning device to a color image forming apparatus main body. In this case, the scanning line inclines or curves, and the slope and curvature change between colors, which appears as a registration error.
As a measure against the registration error, Japanese Patent Laid-Open No. 2002-116394 discloses a method of measuring the curvature of the scanning line by using an optical sensor in the deflection scanning device assembly step, mechanically rotating the lens to adjust the curvature of the scanning line, and fixing the deflection scanning device with an adhesive.
Japanese Patent Laid-Open No. 2003-241131 discloses a method of measuring the slope of the scanning line by using an optical sensor in the step of mounting a deflection scanning device in a color image forming apparatus main body, mechanically inclining the deflection scanning device to adjust the slope of the scanning line, and mounting the deflection scanning device in the color image forming apparatus main body.
Japanese Patent Laid-Open No. 2004-170755 discloses a method of measuring the slope and curvature of the scanning line by using an optical sensor, correcting bitmap image data so as to cancel the slope and curvature, and forming a corrected image. This method electrically corrects a registration error by processing image data. The method disclosed in Japanese Patent Laid-Open No. 2004-170755 can cope with a registration error at lower cost than the methods disclosed in Japanese Patent Laid-Open Nos. 2002-116394 and 2003-241131 because neither the mechanical adjustment member nor the adjustment step in assembly is necessary.
Electrical registration error correction as disclosed in Japanese Patent Laid-Open No. 2004-170755 is divided into correction of each pixel and correction of smaller than one pixel. In correction of each pixel, each pixel is offset in the sub-scanning direction in accordance with the slope and curvature correction amounts. In correction of smaller than one pixel, the tone value of bitmap image data is adjusted in preceding and succeeding pixels in the sub-scanning direction. Correction of smaller than one pixel can eliminate an unnatural step which appears at the offset boundary due to correction of each pixel, and can smooth an image.
Japanese Patent Laid-Open No. 2005-7621 discloses a method of masking a laser output so as not to print in an area other than a printable area corresponding to the printing paper range.
However, the above prior arts pose the following problems.
In electrical registration error correction, which is one of measures against a registration error, electrical correction means modifying, e.g., a rectangular image. If an image is modified to mask a laser output in an area other than the printable area, an effective image area modified in accordance with the image modification amount is masked with the masked area, omitting an image.